Arc discharge lamps comprise an hermetically sealed arc tube and a fill within the arc tube. The fill generally comprises mercury; an inert gas, for starting purposes; and in the more recent art, one or more metal halides to improve the efficacy and color-rendering properties of the lamp. These lamps are well known and frequently employed in commercial usage because of their long life, about 10,000 to 20,000 hours depending on wattage, and their high efficacy. The efficacy of lamps is usually measured in lumens per watt and will be so designated herein. Metal halide arc lamps have efficacies in the neighborhood of 80 to 125 lumens per watt depending on the wattage.
The arc tube in arc discharge lamps is generally made of fused quartz or a high silica glass, e.g., Vycor. The arc tube generally is a straight or arched cylindrical tube having press seals at each end. When the arc tube is a straight cylinder, positional orientation of the arc tube during lamp operation is not essential.
During normal lamp operation, the temperature of the arc tube can range between 500.degree. and 1000.degree. C. and the pressure within the arc tube can range between 1 and 30 atmospheres. When a lamp with a straight arc tube is operated horizontally, convection currents within the arc tube cause the arc discharge to arch or bow upwards resulting in higher temperatures at the upper wall of the arc tube. This phenomenon results in lower lamp efficacy. Also, increased upper-wall temperature results in increased sodium losses and quartz-electrode reactions.
In U.S. Pat. No. 3,858,078, issued Dec. 31, 1974, Koury taught that the efficacy and lumen maintenance of a metal halide arc discharge lamp operating in a horizontal position can be improved by modifying the geometry of the arc tube such that the arc tube is arched to accommodate the bowing of the arc discharge during full temperature and pressure operation of the lamp. The significant improvements disclosed in the Koury patent, which are still current in the art, are based on the principle that arc tube walls should be designed such that they will be substantially parallel and equidistant from the longitudinal axis of the arc discharge during full temperature and pressure operation of the lamp. See also A New Generation of Metal Halide Lamps, by Koury, Gungle, and Waymouth, Journal of the Illuminating Engineering Society, January, 1975.
U.S. Pat. No. 4,056,751, by Gungle, et al, issued Nov. 1, 1977, discloses an improvement on the mentioned Koury patent wherein the electrodes are located approximately midway between the lower wall and longitudinal axis of the arc tube in order to reduce condensation of the metal halides at the ends of the arc tube during operation of the lamp. In this improvement, the upper and lower walls of the arc tube are substantially parallel, each wall having essentially the same degree of curvature.
Japanese Utility Model Publication No. 44-3499, published Feb. 7, 1969, discloses a lamp with an arched arc tube wherein the upper and lower walls of the arc tube are substantially parallel. Other types of arc discharge lamps have employed curved arc tubes. Arc discharge mercury lamps of approximately 60 years ago used curved arc tubes in order to keep separate the two pools of liquid mercury which served as electrodes.
Some capillary arc discharge lamps of approximately 40 years ago had a curved upper arc tube wall in a horizontal-operation lamp to prevent overheating of the middle of the arc tube. These lamps were very heavily loaded in comparison to current commerical metal halide lamps which have a loading of approximately 15 watts/sq. cm. The arc tubes of these capillary lamps had thicker walls and smaller bore than their current counterparts. In such capillary lamps, the arc discharge extended to the walls of the arc tube and was confined thereby. In contrast, the arc discharge of current metal halide lamps is not confined by the walls of the arc tube and the shape of the arc discharge may be affected by convection currents within the arc tube. In U.S. Pat. No. 2,190,657, issued Feb. 20, 1940, Germer discloses an arc tube comprising a curved upper wall and a flat lower wall having an interior bore narrow enough to confine the arc. Germer teaches that a curved upper wall will prevent overheating of the middle of the arc tube. Germer does not teach nor suggest that improved starting capabilities, efficacy, and lumen maintenance can be attained in a lamp having a non-capillary arc tube with a curved upper wall. In the Germer disclosure, the lower wall of the arc tube is flat and not determined by optimization of temperature profiles.
Curved arc tubes are also disclosed in the field of low pressure arc discharge lamps, such as low pressure sodium lamps and fluorescent lamps. It is common for such lamps to employ a U-shaped arc tube in order to conserve the space required for the lamp. Because these lamps are so lightly loaded, the length of the arc tube is much greater than would be required in a metal halide lamp of equivalent or even moderately increased wattage.